Primary amelanotic malignant melanoma of the colon

Malignant melanomas in the gastrointestinal tract are usually metastases from cutaneous melanomas [1], and primary melanomas in the esophagus or anorectal lesions rarely occur. Primary colonic malignant melanoma is extremely rare, with only eight cases previously reported [2]. Although melanomas usually exhibit macroscopic pigmentation, 30 % are amelanotic [3]. Diagnosis of amelanotic melanoma of the gastrointestinal tract by endoscopic examination is difficult owing to its resemblance to gastrointestinal stromal tumor (GIST). We report the case of a patient with amelanotic melanoma of the cecum presenting as a submucosal tumor (SMT), detected by colonoscopy. To our knowledge, this is the first report of primary amelanotic melanoma of the colon.

A 39-year-old woman was referred to our hospital for lower abdominal pain. Colonoscopy revealed an SMT (diameter 20 mm) in the cecum ([Fig. 1]). Endoscopic ultrasonography revealed a hypoechoic SMT derived from the muscularis propria, which was suspected to be a GIST ([Fig. 2]). The patient was offered two possible options: conservative follow-up and surgery. The patient gave informed consent for surgery and laparoscopic ileocaecal resection was carried out. The resected tumor measured 20 × 15 × 10 mm and its cut surface was milky white in color ([Fig. 3]). On histological examination, the tumor cells were spindle shaped with abundant cytoplasm ([Fig. 4 a]). In addition, these cells showed strong positive immunohistochemical staining for HMB-45 ([Fig. 4 b]) but weak positive staining for both smooth muscle antigen and c-kit. These characteristics were consistent with malignant melanoma and the tumor was identified as an amelanotic melanoma. Whole-body computed tomography and positron emission tomography did not reveal any other primary tumors, and thus a diagnosis of primary melanoma of the cecum was established.

Fig. 1 Colonoscopic view of a 20-mm submucosal tumor in the cecum, thought to be a gastrointestinal stromal tumor.

Fig. 2 Endoscopic ultrasonography showing a homogeneous, hypoechoic tumor derived from the muscularis propria (mp).

Fig. 3 The resected tumor had a milky white cut surface and measured 20 × 15 × 10 mm.

Fig. 4 a Histological section showing spindle-shaped tumor cells with abundant cytoplasm (hematoxylin and eosin; magnification × 400). b Strongly positive immunohistochemical staining for HMB-45 (magnification × 400).

Competing interests: None

Endoscopy_UCTN_Code_CCL_1AD_2AC

References

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• 2 Kenney B, Dotto J, Homer R. et al . Primary malignant melanoma of the transverse colon: report of a case and review of the literature.  Int J Surg Pathol. 2007;  15 401-407
• 3 Hilenrand A, Barth T F, Henne-Bruns D. et al . Anorectal amelanotic melanoma.  Colorectal Dis. 2007;  10 612-615